Photodynamic therapy (PDT) is a non-surgical technique for treatment of cancers and other diseases in which administration of a non-toxic photosensitizing agent (a drug that is activated by light), that is uptaken by and retained in a tumor or another tissue to be treated, is followed by non-hazardous irradiation with light of a particular wavelength that generates cytotoxic reactive oxygen species (singlet oxygen) in situ. This technique is more selective than conventional chemotherapy and radiotherapy because of preferential accumulation of photoactivatable compounds to tumor tissue and due to controlled light delivery directed toward the tumor that leads to spatially confined photodynamic effects.
Porphyrins have been employed as the primary photosensitizing agents in clinics. Optimal tissue penetration by light apparently occurs between 650-800 nm, but porfimer sodium (Photofrin®t, a trademark of Axcan Pharma Inc.), the world's first approved photodynamic therapy agent which is obtained from hematoporphyrin-IX by treatment with acids, and has received FDA approval for treatment of esophageal and endobronchial non-small cell lung cancers, absorbs only weakly at about 620 nm, and is a complex and inseparable mixture of monomers, dimers, and higher oligomers. In addition, Photofrin®g and other tested photosensitizing agents suffer from several deficiencies that limit their application, including mainly: (1) relatively weak absorption in the visible spectral range which limits the treatment to shallow tumors; (2) accumulation and long retention of the sensitizer in the patient's skin, leading to prolonged (days to months) skin phototoxicity; and (3) small or even no differentiation between the PDT effect on illuminated tumor and non-tumor tissues. These drawbacks and inherent problems have resulted in large amounts of work devoted to the synthesis of single pure compounds—so-called “second generation” sensitizers—which absorb at long wavelength, have well established structures and exhibit better differentiation between their retention in tumor cells and their retention in skin or other normal tissues.
In the search for appropriate light-sensitive molecules, or photosensitizers, bacteriochlorophyll appears to have some advantages over Photofrin®, the most common photosensitizer for PDT therapy. Bacteriochlorophyll, when illuminated, can cause the light to reach deeper into tissue, thereby being more effective for larger tumors. The spectra, photophysics, and photochemistry of native Bchls have thus made them optimal light-harvesting molecules with clear advantages over other photosensitizing agents presently used or tested in PDT treatment. In particular, these molecules have a very high extinction coefficient at long wavelengths (λmax=760-780 nm, ε=(4-10)×104 M−1 cm−1), where light penetrates deeply into tissues. They also generate reactive oxygen species (ROS) at a high quantum yield (depending on the central metal).
The biological uptake and PDT efficacy of metal-free derivatives of Bchl have been studied with the objective to manipulate the affinity of the sensitizers to the tumor cellular compartment. Cardinal to this approach is the use of highly lipophilic substituents that, on one hand, may increase the accumulation of the drug in the tumor cells but, on the other hand, may difficult its delivery to the tumor cells. In addition, one should avoid accumulation of significant phototoxic drug levels in non-tumor tissues over prolonged periods after administering the drug.
In applicant's previous U.S. Pat. No. 5,726,169, U.S. Pat. No. 5,955,585 and U.S. Pat. No. 6,147,195, a different approach was taken by the inventors. Highly efficient anti-vascular sensitizers, that do not extravasate from the circulation after administration and have short lifetime in the blood, were synthesized. It was expected that the inherent difference between vessels of normal and abnormal tissues such as tumors or other tissues that rely on neovessels, would enable relatively selective destruction of the abnormal tissue. Thus, it was aimed to synthesize Bchl derivatives that are more polar and hence have better chance to stay in the vascular compartment, where they convey the primary photodynamic effect. Manipulation at the 17-propionic acid residue site of the native Bchl provided conjugates with various residues such as amino acids, peptides or proteins, which enhance the sensitizer hydrophilicity. The vascular targeting activity of one of these derivatives, bacteriochlorophyll-serine, was studied as well as its fast clearance from the circulation and the entire animal body, lack of skin phototoxicity and high curative potential (Rosenbach-Belkin et al, 1996; Zilberstein et al., 1997; Zilberstein et al., 2001). Yet, these Mg-containing compounds were found unsuitable for pharmaceutical use due to their low stability on prolonged storage.
To increase the stability of the Bchl derivatives, the central Mg atom was replaced by Pd in the later applicant's PCT Publication WO 00/33833 and corresponding U.S. Pat. No. 6,569,846. This heavy atom was previously shown to markedly increase the oxidation potential of the Bchl macrocycle and, at the same time, to greatly enhance the intersystem-crossing (ISC) rate of the molecule to its triplet state. The metal replacement was performed by direct incorporation of Pd2+ ion into a Bpheid molecule, as described in WO 00/33833. The first Pd-substituted Bchl derivative, palladium-bacteriopheophorbide or Pd-Bpheid (Tookad®, a trademark of Steba Biotech), was found highly effective against various solid tumors in pre-clinical studies (Schreiber et al., 2002; Gross et al., 2003; Koudinova et al., 2003; WO 03/094695) even against tumors comprising resistant tumor cells (Preise et al., 2003). The antivascular activity of Pd-Bpheid enabled destruction of the prostetic glandular tissue in dog models without compromising their continence (Chen et al., 2002). Phase I/II clinical trials proved that Pd-Bpheid is safe for use in the photodynamic therapy of prostate cancer in patients that failed radiation therapy (Elhilali, 2004) and induces necrosis and PSA (prostate specific antigen) reduction of vascularized glandular tissue in prostate patients treated with therapeutic light and drug doses (Trachtenberg, 2003).
Because of its low solubility in aqueous solutions, the clinical use of Pd-Bpheid requires the use of solubilizing agents such as Cremophor that may cause side effects at high doses. This lead the inventors to conceive a new family of Bchl derivatives, described in PCT/IL03/00973 (WO 2004/045492), consisting of the Bchlorin macrocycle containing a di- or trivalent central metal atom and at least two anionic residues. These anionic Bchl compounds can be administered intravenously after solubilization in aqueous solutions with no added excipients. Their short life-time in the circulation, combined with their relatively fast action and highly efficient anti-vascular activity, show their potential as antivascular PDT agents. In fact, one of these anionic Bchl derivatives is presently in preclinical studies for PDT of age-related macular degeneration (AMD) and liver tumors, e.g. hepatoma.
DE 10154436 describes pyrobacteriopheophorbide compounds for use in photodynamic therapy, in which at least one of the keto groups at position 3a or 131 of the porphyrin system is derivatized to a corresponding imine.
WO 03/028629 describes chlorophyll derivatives that may contain positively charged ammonium or iminium groups for photodynamic therapy or diagnosis.
WO 03/028628 describes tetrapyrrolic macrocycles that are substituted by at least one functional group that comprises a carbamate group of the formula —OCON< or —OCON═C< and optionally contain positively charged ammonium or iminium groups, for photodynamic therapy or diagnosis. Although the general formulas disclosed in said publication include bacteriochlorophyll derivatives, it is to be noted that specific bacteriochlorophyll derivatives have not been disclosed nor does the specification teaches the preparation of bacteriochlorophyll derivatives.
It would be highly desirable to provide new bacteriochlorophyll derivatives that would be stable and would have enhanced affinity to endothelial cells for use in photodynamic therapy and, particularly, in vascular targeted phototherapy (VTP).